package com.hit.basmath.learn.others;

import com.hit.common.ListNode;

/**
 * 25. Reverse Nodes in k-Group
 * <p>
 * Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.
 * <p>
 * k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is.
 * <p>
 * Example:
 * <p>
 * Given this linked list: 1->2->3->4->5
 * <p>
 * For k = 2, you should return: 2->1->4->3->5
 * <p>
 * For k = 3, you should return: 3->2->1->4->5
 * <p>
 * Note:
 * <p>
 * Only constant extra memory is allowed.
 * You may not alter the values in the list's nodes, only nodes itself may be changed.
 */
public class _25 {
    public ListNode reverseKGroup(ListNode head, int k) {
        ListNode curr = head;
        int count = 0;
        while (curr != null && count != k) { // find the k+1 node
            curr = curr.next;
            count++;
        }
        if (count == k) { // if k+1 node is found
            curr = reverseKGroup(curr, k); // reverse list with k+1 node as head
            // head - head-pointer to direct part,
            // curr - head-pointer to reversed part;
            while (count-- > 0) { // reverse current k-group:
                ListNode tmp = head.next; // tmp - next head in direct part
                head.next = curr; // preappending "direct" head to the reversed list
                curr = head; // move head of reversed part to a new node
                head = tmp; // move "direct" head to the next node in direct part
            }
            head = curr;
        }
        return head;
    }
}
